Silver halide photographic emulsions are well known in the art. It is known in the art that silver halide emulsions can be spectrally sensitized to increase the photographic response to specific wavelengths of actinic radiation.
Spectral sensitization of photographic emulsions to blue and ultraviolet radiation is a widely recognized desire in the art. This desire is due, in part, to the superior resolution which can be obtained in blue and ultraviolet sensitive medical X-ray films relative to green or red sensitive medical X-ray films.
Zeromethine merocyanine dyes have been shown to be effective for spectral sensitization of tabular grains to blue light as detailed in U.S. Pat. No. 5,108,887. The compounds taught employ the methyl derivative at the benzothiazole nitrogen. Replacing the methyl group of the benzothiazole nitrogen with an ethyl or sulfobutyl group was shown to be detrimental to the function of the dye. Furthermore, the dyes of U.S. Pat. No. 5,108,887 are only effective for tabular grains and not for other important grain morphologies such as cubic. Based on the teachings of U.S. Pat. No. 5,108,887 a skilled artisan was led to the conclusion that the methyl substituent was critical and that substitution thereof was not advantageous.
The dye art is replete with teachings on the similarities of dye substituents. U.S. Pat. No. 4,028,355, for example, indicates the equivalency of alkyl ammonium, alkyl sulfonate, and alkyl phosphonate on trimethine dyes. The similarities do not translate to zeromethine dyes as indicated by the marked difference between methyl and ethyl substitution and the inapplicability of the butyl sulfonate as taught in U.S. Pat. No. 5,108,887.
Described herein is a novel zeromethine merocyanine dye comprising an alkyl ammonium substituent which surprisingly renders the dye suitable as a spectral sensitizer for silver halide photographic elements. This novel dye provides adequate photographic response at a low level of fog and without the appearance of staining due to residual dye remaining in the film element after photographic processing. Furthermore, the dye is suitable for spectral senstization to both blue and ultraviolet light and for use with various grain morphologies.